Smash's whole identity is being a crossover fighter. The crossover aspects define and influence every bit of gameplay - The movesets, the stages, the collectables, so on and so forth. It would be window dressing if it was just Marios wrapped around a generic shotoclone moveset slugging it out on a generic flat plan colored vaguely based on Mario stages, but it's not; It's specifically Mario with his Fireballs, Cape, and FLUDD, fighting on the specific layout of Princess Peach's Castle, with very specifically a Banzai Bill or behemoth bill as they want to call it now crashing into the stage. The only "window dressing" character is Captain Falcon reusing the placeholder moveset from when Smash was still The Dragon King, but even then, we wouldn't love him so much if they didn't make him scream "FALCON PAWNCH" every time he used the big strong punchy move.
So in short, good gameplay isn't enough to make a truly great game, you need to give it the sauce if you want it to stand out.